Self-tapping and self-retaining, screw thread insert

ABSTRACT

Screw thread insert of the self-tapping type, in the form of an externally threaded tubular sleeve or a solid-shank stud. When of the sleeve type it has an open bore extending from end to end, with at least the back end section of its bore provided with internal threads for anchoring threaded members therein. When of the stud type its front or tip end section is counter-bored or socketed. In either type this screw thread insert embodies an effective self-retaining characteristic in the form of at least one external localized and longitudinal camber area extending back from its tip end and into its midsection with this camber area being provided along its crown with a generally longitudinal kerf channel defining by its trailing side a thread cutting edge for forming a female receptive thread in the wall of a parent receiving body pilot hole. The kerf channel interrupts a full diameter external thread turn of the midsection in a hump therein to form at the severed leading end of this thread turn a cutting tip at a radius slightly greater than the normal thread radius. A following full diameter thread turn has a similar uninterrupted hump therein to provide an insert-retaining means.

United States Patent 1 1 Kraus et a1.

[ 1 Feb. 18, 1975 1 1 SELF-TAPPING AND SELF-RETAINING,

SCREW THREAD INSERT [73] Assignee: Mite Corporation, Danbury, Conn.

[22] Filed: Aug. 22, 1973 [21] Appl. No.: 390,428

[52] US. Cl 85/32 R, 85/47, 151/22 [51] Int. C1...Fl6b 25/00, F16b 37/00, F16b 39/30 [58] Field of Search 85/47, 41, 1 L, 32 R; 10/152 T; 408/230, 226; 151/22, 14 R [561 References Cited UNITED STATES PATENTS 2,278,377 3/1942 (Took 85/47 2,455,885 12/1948 Theurcr..v 85/47 X 3,174,521 3/1965 Harvey 151/14 R 3.251.080 5/1966 Sharon 111/152 T FOREIGN PATENTS OR APPLICATIONS 682.980 3/1964 Canada 85/47 564,100 10/1923 France 85/47 843,129 8/1960 Great Britain t 85/47 896595 5/1963 Great Britain 85/47 Primary Examiner-Ramon S. Britts Attorney, Agent, or Firm-Watson Leavenworth Kelton & Taggart 1 1 ABSTRACT Screw thread insert of the self-tapping type, in the form of an externally threaded tubular sleeve or a solid-shank stud. When of the sleeve type it has an open bore extending from end to end, with at least the back end section of its bore provided with internal threads for anchoring threaded members therein. When of the stud type its front or tip end section is counter-bored or socketed. In either type this screw thread insert embodies an effective self-retaining characteristic in the form of at least one external localized and longitudinal camber area extending back from its tip end and into its midsection with this camber area being provided along its crown with a generally longitudinal kerl' channel defining by its trailing side a thread cutting edge for forming a female receptive thread in the wall of a parent receiving body pilot hole. The kerl channel interrupts a full diameter external thread turn of the midsection in a hump therein to form at the severed leading end of this thread turn a cutting tip at a radius slightly greater than the normal thread radius. A following full diameter thread turn has a similar uninterrupted hump therein to provide an insertretaining means.

7 Claims, 4 Drawing Figures 1 SELF-TAPPING AND SELF-RETAINING, SCREW THREAD INSERT BACKGROUND AND SUMMARY The present invention relates to self-tapping inserts equipped with retaining means. Such screw thread inserts, when of the tubular sleeve type has at least the back end section of its through bore internally threaded for screw-anchoring purposes. When of the stud-shank type its tip end section may be counter bored. In either form its exterior is provided with cutting edges which will cut internal threads in the walls of cylindrical pilot holes formed in parent receiving bodies of material softer than those of the insert.

It has been proposed in Theurer U.S. Pat. No. 2,455,885 of Dec. 7, 1948 to provide different embodiments of such self-tapping sleeve inserts. In one embodiment the tapered tip end section is provided with circumferentially-spaced, longitudinal slots which break inward through the sleeve body wall into the internally-threaded bore to form trailing flanks which have cutting edges and intervening resilient fingers or tongues that are to serve as gripping retention means. In this form chips which are cut from the parent body pilot hole wall enter through the slots into the internally-threaded bore to foul the threads thereof. Also, due to the inward flexure of the fingers or tongues, which provide the cutting edges on their trailing flanks during the rotary advancing thread cutting action, there is tendency to avoid cutting full diameter thread turns and intervening root turns, so that the following full diameter and uninterrupted external thread turns of the sleeve insert may have too close and uncontrollable interference fit with the parent body female threads which tends to develop a degree of jamming that hazards seating tool breakage, which would be most likely in metal parent materials. Or the sleeve insert may tend to be too tightly clamped by the flexed tongues or fingers upon the seating tool for effective and proper back-out of the tool. In the FIG. 4 embodiment of Theurer it is proposed that the cutting edges be provided by generally longitudinally kerf channels, which will avoid passage through to the internal thread of fouling parent body chips, but the patentee recognizes in the specification that there is no adequate retention deflection of the strips of the insert wall between these channels.

The Harvey U.S. Pat. No. 3,174,521 of Mar. 23, l965 teaches a self-locking screw which has an externally-threaded solid shank provided with a transverse hole into which a tapered pin is driven transversely to bulge outwardly the areas of the thread turns on opposite sides of this transverse hole, for serving as locking means to provide interference fit with the female thread in a receptive parent body or nut. It does not propose any self-tapping feature in association therewith nor a sleeve insert equipped with a combination of cooperative self-tapping and self-retaining means.

An object of the present invention is to provide a practical self-tapping and self-retaining, screw thread sleeve or stud-shank insert which combines these features in a unique manner by equipping its exterior at one or more places with a generally longitudinal localized area that is distorted transversely outward slightly to project radially to a limited degree for forming a composite camber in which the sections of the external thread turns that cross this camber area are humped. A

forward zone of such a camber area is provided with a generally longitudinal kerf channel or flute which interrupts forward thread turn hump portions there to produce interrupted thread turns as effective cutting edges. These cutting edges will form the receptive female thread turns in the parent body receptive hole wall upon thread advancing rotation of the sleeve body, while retaining uninterrupted a hump portion of at least one following full diameter external thread turn to serve as an effective and controlled retaining means.

Another object is to provide these cooperative features in an improved self-tapping and self-retaining sleeve insert without distorting the internally-threaded bore of the sleeve body so as to assure proper threaded anchorage of a screw or stud therein.

The present screw thread insert is characterized by a body of relatively hard rigid material, such as, for example, carbon or corrosion resisting or plated steel, brass, relatively hard aluminum compositions, or other suitable materials that are appreciably harder than the parent body material. The appreciably softer and relatively rigid parent body material may, for example, be cast or wrought aluminum or zinc alloys, cast magnesium, or relatively rigid plastics that are appreciably softer than the insert material.

The insert body has a leading tip end, a tip end section, a trailing back end and an intervening midsection. It is provided with an external male thread of any conventional or desired convenient style from the tip end back through the midsection at least to a point that normally will be located in the parent body receptive hole when fully seated therein. The external thread turns on the midsection are of full diameter while those on the tip end section are taper chamfered to reduce the thread depth forward to the tip end of the insert body.

It is important to the efficient practice of the present invention that at the cutting tip end of at least one interrupted full diameter external thread turn in the insert midsection, the radius be slightly greater than the nominal radius of such diameter thread turn in the portions of the insert which have not been transversely increased slightly by the provision of such a camber area. Thus the generally longitudinal kerf channel should extend along the crown of such camber area into and through at least the first full diameter external thread turn immediately back of the last thread turn of the tapered tip end section, so that the cutting tip of this first interrupted turn of the midsection on the trailing side of the kerf channel is located substantially at this slightly greater radius from the insert axis. Such camber area may be so shaped transversely that the crest of its crown may have a relatively appreciable width. It is the function of this raised cutting tip as it is helically advanced in the female thread that is precedently cut progressively by the cutting thread turn tips in the tapered section to offset the inherent characteristic of the parent body material to spring back very slightly so as to close in sufficiently to form such a tight interference fit at the humped thread portions in each such camber area that may cause enough binding to hazard breakage of the rotary seating tool and incompletion of the insert seating.

It is also fundamentally important to an effective retention of the fully seated insert that at least one full di' ameter thread turn in the midsection have a humped portion extending across such camber area that is uninterrupted, i.e., not severed by such kerf channel th'ere to form such a thread tip cutting edge, so that this humped thread portion and flanking humped roots together form a camber portion shaped like the arc of a transversely ribbed ellipse to act as a securing spring against the circular opposed internally-threaded wall of the parent body tapped hole. Without such a spring section the insert will cut a female thread in the parent body receptive hole, but its backout torque will be undesirably low. By the inclusion of more than one of the full diameter thread turns within the camber area and omission of the kerf channel interruption thereof the retentive characteristic can be selectively increased incrementally, each such added turn imposing a readily determinable increment of the amount of retention desired in a particular selection of the materials of the sleeve insert and parent body, the particular size of the sleeve insert and the duty it is to perform, as well as the degree of the camber height chosen. Whether the insert body be a tubular sleeve or a solid-shank stud having a counterbored leading end, so that the leading end section of the insert body has a generally tubular wall, the one or more camber areas should be located on this tubular wall in order to realize fully the desired retention characteristic.

The amount of retention desired can also be increased controllably by increasing selectively the number of localized camber areas that are provided. It may be desired for assuring a balanced insertion that is substantially free from insert cocking tendency to provide the insert body with a plurality of such camber areas located at substantially equal radial angles apart, e.g., 90, l and 180. So long as one of the localized camber areas is provided with a generally longitudinal kerf channel to form in at least one full diameter thread turn an interruption defining a greater radius cutting tip substantially at the crest of such camber area hump portion of this thread turn, then less than the full number or none of the additional camber areas need be provided with such a generally longitudinal kerf channel.

The degree of the camber height at the crest of the kerf-interrupted full diameter thread turn, i.e., the height of the thread turn hump thereat over the flanking circular thread portions of this turn, should be quite small or slight, e.g., of the order of about 0.001 inch to about 0.010 inch, depending upon the size of the insert. The greater the diameter of the insert the greater may be the slight difference between the radius of the circle described by the thread cutting tip of the full diameter turn at the crest of the hump therein and the radius of the flanking circular portions of this thread turn, within the approximate range indicated.

It may also be desired to assure a substantial balance between the torques required to effect the thread cutting action of thread turn tips in the trailing sides of such kerf channels when a plurality thereof are provided. For example, a pair thereof desirably may be provided at diametrically opposite points, or at quarter points, or at 120 apart, etc. An effective arrangement has been determined to be the provision of three such localized camber areas located 120 apart with one thereof equipped with such a generally longitudinal kerf channel, and the additional provision of a similar kerf channel located diametrically opposite the firstmentioned one outside of any such localized camber area.

The localized and longitudinal raised camber areas in the externally threaded surface of the insert body may be formed in any suitable manner which is apparent to one skilled in this art. For example, the external male thread may be formed by a tool bit or thread chaser as the insert blank is axially rotated, with this tool being carried by a support which is caused by suitable means, such as a cam, to recede transversely where such camber areas are to be produced. For this purpose the tool support may be in the form of a rocker arm having a cam follower riding against a suitably lobed rotating arm. The pitch of the external threads as they are being cut may be dictated by translation along the insert blank axis relative to the tool support or vice versa.

Other objects of the invention will in part be obvious and will in part appear from reference to the following detailed description taken in connection with the accompanying drawing, wherein like numerals identify similar parts throughout and dimensions of certain parts are considerably exaggerated for clarity in understanding, and in which:

FIG. 1 is an elevational side view, to an enlarged scale, of an embodiment of the self-tapping and selfretaining, screw thread sleeve insert of the present invention;

FIG. 2 is a tip end view of the sleeve insert depicted in FIG. 1;

FIG. 3 is a sectional detail of the sleeve insert of FIGS. 1 and 2 longitudinally through the kerf channel shown in FIG. 1 and that depicted on the right side of FIG. 2; and

FIG. 4 is a tip end view, similar to FIG. 2, of another embodiment of sleeve insert of the present invention.

It will be seen from FIGS. 1 to 3 inclusive that the embodiment of the sleeve insert 5 illustrated therein consists of a sleeve body 6 having a helical external male thread 7, and with its axial bore provided with an uninterrupted helical female thread 9. This internallythreaded sleeve body has a leading tip end 10, a tip end section 11, a trailing back end 12, and an intervening midsection 13. The successive external thread turns on the midsection are of full diameter and those on the tip end section 11 are taper chamfered progressively to reduce the thread depth toward the tip end 10, with this chamfering being of conventional form such as at an angle a of about l2 /z. The external male thread 7 has its forward thread turns interrupted to produce a trailing cutting edge which includes severed tip ends of these interrupted thread turns for rotationally cutting a mating female thread into the wall of a receptive pilot hole in a parent body of rigid material which is relatively softer than the hard rigid material of the insert sleeve body 6, as will be explained later.

A generally longitudinal localized area, indicated at 14-1 in FIGS. 1 and 2, is provided on one exterior side of the sleeve body 6 from the tip end 10 back into the midsection 13 beyond portions of at least the first two thread turns of full diameter. In the FIGS. 1 to 3 inclusive embodiment three such generally longitudinal localized areas are provided at 14-l, 142, and 14-3 at substantially equal radial angles apart, e.g., At least the first two thread turns of full diameter on the midsection 13 back of the chamfered tip end section 1 1 are included in this generally longitudinal localized area which projects radially outward to a quite limited degree to form a composite camber area in which the external thread turns which cross this localized camber area have hump portions provided with smooth transitions on the rotationally leading and trailing sides of this area with the adjacent circular portions of these thread turns flanking it, as will be best understood from FIG. 2.

One of these localized camber areas, by way of illustration that at 114-3, has a forward zone provided with a generally longitudinal and external threadinterrupting kerf channel or flute 15 extending across the external thread turns in this zone from a point rearward of at least the first full diameter turn in this midsection to the tip end without breaking inward through the wall of the sleeve body 6. The rotationally trailing side 16 of this kerf channel and the severed ends of the external thread turns therein provide cutting edges which cut the female thread turns in the parent body receptive hole wall upon thread advancing rotation of the insert body thereinto. Since this kerf channel I5 interrupts a full diameter thread turn it provides, at the trailing severed end 17 thereof, a cutting tip on a radius which is slightly greater than the radius of the circular portions of this full diameter turn, which is the radius of the full diameter turns of the midsection back of the localized camber areas, and this slightly greater radius is substantially that of the crests of the hump portions of all uninterrupted full diameter thread turns which cross such camber areas. As has been previously indicated the height of such thread turn humps over the flanking parallel thread portions of such thread turns is quite small or slight, e.g., of the order of about 0.001 inch to about 0.010 inch, depending upon the size of the sleeve insert and desired end use.

By so confining the thread-interrupting kerf channel 15 to a forward zone of such localized camber area there is provided across the latter immediately back of this kerf channel at least one uninterrupted full diameter thread turn of the midsection which has an uninterrupted crossing hump portion, so that the latter will serve with the female threads cut into the parent body receptive hole a retaining means upon full seating of the sleeve insert into the parent body.

In the embodiment of FIGS. 1 to 3 inclusive it will be noted that the localized longitudinal camber areas 14-1 and 14-2 are free of such a kerf channel so that each provides full diameter thread turn humps to serve as cooperative retaining means. In this illustrated embodiment there may, for example, be about nine such retaining thread turn humps, depending upon the thread pitch.

It will also be noted from FIG. 2 that a similar kerf channel 150 is provided diametrically opposite that at 15 which interrupts thread turn portions in the localized camber area 14-3, but that this second kerf channel does not interrupt thread turn humps. It does provide a trailing cutting edge and cutting thread turn tips on the smaller radius to supplement the thread cutting action of the kerf channel 15 for balancing to a desired degree the thread cutting torque.

It will be noted from FIGS. 1 and 2 that the bottom surface 18 of each of the kerf channels 15 and 150 is substantially flat in the tangential direction and concaved in the longitudinal direction to run out to a full diameter thread turn for causing cut parent body material to break up into chips which may be readily removed from a receptive blind hole in the parent body. It will further be noted that each of the kerf channels provides a positive rake angle in the trailing cutting sides thereof to assure an efficient thread cutting action. The shape of each of these kerf channels is such that, desirably, chips are formed ahead of the advancing insert similar to the action of a spiral point tap.

It is to be understood that as to the function of the uninterrupted internal thread in the through bore of such a seated sleeve insert for threaded anchorage therein of a screw or stud this need not be provided throughout the extent of this bore. For many services it may be sufficient if only the portion of this bore which extends from the trailing back end of the insert appreciably forward to the general vicinity of the camber area or areas be provided with such an internal thread.

The shape of the kerf channel and the number thereof are optional within the scope of the present invention. Although the illustrations in FIGS. 1 to 3 inclusive have been found to be ideal, FIG. 4 illustrates another embodiment of the invention which satisfies the requisite characteristics. The sleeve insert 50 of FIG. 4 is provided with a pair of the diametrically located localized longitudinal camber areas 14-1 and 14-2 each of which is provided with a kerf channel 1500 to form on its trailing side a cutting edge that includes a severed cutting thread tip substantially at the medial crest of a full diameter thread hump which extends across the localized camber area in which such kerf channel is formed.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Having described our invention, what we claim as new and desire to secure by Letters Patent is the novel subjects matter defined in the following claims:

1. A self-tapping and self-retaining, screw thread insert comprising an externally threaded cylindrical body of relatively hard rigid material having at least a hollow leading end portion defined by a tubular wall with external thread turns being interrupted for rotationally cutting a mating female thread into the wall of a receptive hole in a parent body of relatively softer rigid material; said insert body having a leading tip end, a tip end section, a trailing back end and an intervening midsection with the successive external thread turns on the latter being of full crest diameter and those on the tip end section having the crests thereon taper chamfered progressively to reduce the thread depth toward the tip end; characterized by the provision of l. a generally longitudinal localized area on at least one exterior side of the tubular wall of said hollow leading end portion from said insert tip end back into said insert midsection beyond portions of at least the first two thread turns of full diameter on the latter and which extends transversely outwardly to a limited degree beyond the remaining corresponding portions of the insert, said area forming a composite camber in which the sections of the external thread turns crossing this localized camber area are humped providing thread turn hump por tions projecting radially outwardly beyond adjacent thread turn portions and having smooth transitions on the rotationally leading and trailing sides of said camber area with the adjacent circular portions of these thread turns, the thread turns on said midsection, except for said hump portions, being of uniform crest and root diameters;

2. said localized camber area having a forward zone provided with a generally longitudinal and external thread-interrupting kerf channel extending forward across the external thread turns in this zone from a point rearward of at least the first full diameter turn in said midsection to said tip end without breaking inward through the wall of said sleeve body, whereby the rotationally trailing side of said kerf channel and the severed ends of the external thread turns therein provide cutting edges to form the female thread turns in the parent body receptive hole wall upon thread advancing rotation of said insert body thereinto with at least one uninterrupted full diameter thread turn of said midsection having such a hump portion crossing said camber area to serve with the female threads of said parent body an insert retaining means upon full seating of said insert into the parent body.

2. The self-tapping and self-retaining, screw thread insert of claim 1 characterized by the provision of a plurality of such localized camber areas that are spaced circumferentially at substantially equal radial angles to assure maintenance of coaxial alignment of said insert and the parent body receptive hole during rotational thread-cutting advance of the former into the latter.

3. The self-tapping and self-retaining, screw thread insert of claim 2 characterized by only one of said localized camber areas being provided with said kerf channel.

4. The self-tapping and self-retaining, screw thread insert of claim 3 characterized by said localized camber areas being spaced circumferentially at radial angles which locate them other than diametrically, a similar kerf channel being provided diametrically opposite the first kerf channel outside of such camber areas.

5. The self-tapping and self-retaining, screw thread insert of claim 2 characterized by the location of a pair of such localized camber areas at diametrically opposite places with each provided with one ofa pair of said kerf channels that are located diametrically opposite each other.

6. The self-tapping and self-retaining, screw thread insert of claim 1 characterized by said kerf channel having a bottom surface which is substantially flat in the tangential direction and concaved in the longitudinal direction to run out to a full diameter thread turn for causing cut parent body material to break up into readily removable chips.

7. The self-tapping and self-retaining, screw thread insert of claim 1 characterized by said body being that of a sleeve insert having a through bore extending forward from the trailing back end to the tip end with at least the portion of this bore which extends appreciably forward to the general vicinity of said camber area having an uninterrupted internal thread. 

1. A self-tapping and self-retaining, screw thread insert comprising an externally threaded cylindrical body of relatively hard rigid material having at least a hollow leading end portion defined by a tubular wall with external thread turns being interrupted for rotationally cutting a mating female thread into the wall of a receptive hole in a parent body of relatively softer rigid material; said insert body having a leading tip end, a tip end section, a trailing back end and an intervening midsection with the successive external thread turns on the latter being of full crest diameter and those on the tip end section having the crests thereon taper chamfered progressively to reduce the thread depth toward the tip end; characterized by the provision of
 1. a generally longitudinal localized area on at least one exterior side of the tubular wall of said hollow leading end portion from said insert tip end back into said insert midsection beyond portions of at least the first two thread turns of full diameter on the latter and which extends transversely outwardly to a limited degree beyond the remaining corresponding portions of the insert, said area forming a composite camber in which the sections of the external thread turns crossing this localized camber area are humped providing thread turn hump portions projecting radially outwardly beyond adjacent thread turn portions and having smooth transitions on the rotationally leading and trailing sides of said camber area with the adjacent circular portions of these thread turns, the thread turns on said midsection, except for said hump portions, being of uniform crest and root diameters;
 2. said localized camber area having a forward zone provided with a generally longitudinal and external thread-interrupting kerf channel extending forward across the external thread turns in this zone from a point rearward of at least the first full diameter turn in said midsection to said tip end without breaking inward through the wall of said sleeve body, whereby the rotationally trailing side of said kerf channel and the severed ends of the external thread turns therein provide cutting edges to form the female thread turns in the parent body receptive hole wall upon thread advancing rotation of said insert body thereinto with at least one uninterrupted full diameter thread turn of said midsection having such a hump portion crossing said camber area to serve with the female threads of said parent body an insert retaining means upon full seating of said insert into the parent body.
 2. said localized camber area having a forward zone provided with a generally longitudinal and external thread-interrupting kerf channel extending forward across the external thread turns in this zone from a point rearward of at least the first full diameter turn in said midsection to said tip end without breaking inward through the wall of said sleeve body, whereby the rotationally trailing side of said kerf channel and the severed ends of the external thread turns therein provide cutting edges to form the female thread turns in the parent body receptive hole wall upon thread advancing rotation of said insert body thereinto with at least one uninterrupted full diameter thread turn of said midsection having such a hump portion crossing said camber area to serve with the female threads of said parent body an insert retaining means upon full seating of said insert into the parent body.
 2. The self-tapping and self-retaining, screw thread insert of claim 1 characterized by the provision of a plurality of such localized camber areas that are spaced circumferentially at substantially equal radial angles to assure maintenance of coaxial alignment of said insert and the parent body receptive hole during rotational thread-cutting advance of the former into the latter.
 3. The self-tapping and self-retaining, screw thread insert of claim 2 characterized by only one of said localized camber areas being provided with said kerf channel.
 4. The self-tapping and self-retaining, screw thread insert of claim 3 characterized by said localized camber areas being spaced circumferentially at radial angles which locate them other than diametrically, a similar kerf channel being provided diametrically opposite the first kerf channel outside of such camber areas.
 5. The self-tapping and self-retaining, screw thread insert of claim 2 characterized by the location of a pair of such localized camber areas at diametrically opposite places with each provided with one of a pair of said kerf channels that are located diametrically opposite each other.
 6. The self-tapping and self-retaining, screw thread insert of claim 1 characterized by said kerf channel having a bottom surface which is substantially flat in the tangential direction and concaved in the longitudinal direction to run out to a full diameter thread turn for causing cut parent body material to break up into readily removable chips.
 7. The self-tapping and self-retaining, screw thread insert of claim 1 characterized by said body being that of a sleeve insert having a through bore extending forward from the trailing back end to the tip end with at least the portion of this bore which extends appreciably forward to the general vicinity of said camber area having an uninterrupted internal thread. 